The present invention relates to a motor controller and an electric power steering system.
An electric power steering system described in, for example, JP-A-2004-328814 has hitherto been known as a system for reducing a torque ripple in a motor which assists steering operation performed by a steering wheel of a vehicle. The electric power steering system changes a compensating current computed by a toque ripple compensation section so as to prevent occurrence of a torque ripple in a motor in accordance with the rotational speed of the motor as well as in accordance with a load imposed on the motor. As a result, a current control section provides the motor with an appropriate compensating current even in a case where the rotational speed of the motor changes from moment to moment according to a driving condition besides the case of occurrence of variations in the load imposed on the motor, so that occurrence of a torque ripple can be reduced sufficiently.
For instance, as shown in FIG. 8, the current control section provides the motor with the compensating current computed by the torque ripple compensation section, thereby compensating for a torque T1 which includes a torque ripple and which is acquired before compensation (indicated by a solid line in FIG. 8) (hereinafter called an “uncompensated torque”) in such a way that occurrence of a torque ripple is reduced sufficiently as in the case of torque T2 (indicated by a broken line in FIG. 8) acquired after compensation (hereinafter called a “compensated torque”).
In JP-A-2004-328814, consideration is given solely to the case where a computing period of the torque ripple compensation section coincides with a control period of the current control section; however, no consideration is given to a case where the computing period of the torque ripple compensation section differs from the control period of the current control section. In a commonly-adopted microcomputer except a high-performance microcomputer, the computing period of the torque ripple compensation section becomes longer than the control period of the current control section because of restrictions on a resource of the microcomputer.
In the case where the computing period of the torque ripple compensation section is longer than the control period of the current control section, an offset arises in electrical angles as the speed of rotation of the motor increases, and control is performed while the torque ripple compensation and the current control are out of phase with each other. Therefore, an effect for reducing a torque ripple is diminished. Moreover, when the rotational speed of the motor increases further, it may be the case where the torque ripple (a torque ripple achieved after compensation) of the motor compensated for in accordance with the compensating current computed by the torque ripple compensation section will become worse than the torque ripple acquired before compensation.
For instance, as shown in FIG. 8, even if a compensating current computed as mentioned above is provided to the motor in a case where the computing period of the torque ripple compensation section is longer than the control period of the current control section, a torque ripple in a compensated torque T3 (indicated by a dashed line shown in FIG. 8) will become worse than the torque ripple in the uncompensated torque T1 when the rotational speed of the motor increases to a predetermined speed or more.